Nanomechanical Resonators

Nanoscale electromechanical systems (NEMS) as well as optomechanical and magneto-mechanical systems hold great promise for enabling ultrasensitive detection of displacements and forces. NEMS resonators fabricated out of layered semiconductors exhibit mechanical resonances up to GHz frequencies and have the potential to detect, e.g. mass changes down to zeptograms (10^-21g), and hence also the binding of individual (macro-)molecules. They have been explored as highly non-linear resonators and as single electron shuttles in the Kotthaus group with theoretical support of the Zwerger group for several years. Present research is focused on more efficient excitation of such mechanical resonators by electrodynamic and optical excitation mechanisms. By trying to identify the causes of damping via scanning probe techniques employed down to low temperatures an improved understanding and consequently a reduction of damping mechanisms is anticipated.

NEMS resonators employing individual carbon nanotubes (CNT) are explored in the Kotthaus and Wixforth groups, with potential applications in biosensing. Since CNTs can also act as molecular transistors, such experiments are intimately related to studies of the electronic and optoelectronic behavior of individual molecules discussed below. Whereas the Kotthaus group will focus on the electromagnetic excitation of suspended CNT resonators, the Wixforth group will employ surface acoustic wave (SAW)-driven fluidics to align CNTs with prefabricated electrodes. SAW will also be used to excite their mechanical resonances via either multiphonon excitation or a turnstile-like movement of the suspension points of the CNT-NEMS. Such CNT-NEMS have potential applications as environmental sensors, as CNTs sensitively react to molecular adsorption both in their electrical conductance as well as their mechanical response. Both groups will also study the low-temperature quantum nature of suspended CNTs. Here the Kotthaus group will primarily focus on the influence of the electron-phonon interaction on electronic conduction, whereas the Wixforth group plans to explore the regime of quantized mechanical motion expected in such resonators.

The recently established Grundler group plans to investigate nanomechanical spin transducers. They consist of a nanomagnet or magnetic/nonmagnetic point contact integrated into a nanomechanical device. Such systems are based on the hybrid NEMS already developed in the Kotthaus group. The Grundler group will couple spin currents to the mechanical motion via spin-transfer torque. The eigenfrequency of the spin system can in particular be tuned to be in or off resonance with the characteristic frequency of the nanomechanical oscillator by external means. Thus resonant and non-resonant magnetomechanical coupling will be explored. A visionary goal is to transform spin currents into mechanical motion and vice versa. Nanomechanical spin transducers are also expected to find applications in spin electronics such as those pursued in research area A.